CS341 Notes

file descriptor:

0 → standard input (stdin)
1 → standard output (stdout)
2 → standard error (stderr)

unistd.h

It declares many system call interfaces for Unix-like operating systems (Linux, macOS, BSD, etc.).

Useful functions:

#include <string.h> strlen()

open(...)

flags → control the behavior of opening the file (read/write mode, create, truncate, append, etc.).

mode_t → sets the permissions of the file only when it’s created.

How to read a file

#include "source.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
// read a file line by line and then identify a specific text in ""
// remember to free() after using sscanf's buffer
void find_movie_score() {
    char* buffer = NULL;
    size_t capacity = 0;
    FILE* fp = fopen("movies.csv", "r");
    int year;
    int score;
    char movie[30];
    getline(&buffer, &capacity, fp);
    while (getline(&buffer, &capacity, fp) != -1) {
        // Read up to 29 characters, stopping if a " is seen.
        // in this case 29 means 1 character less than the 30 length long buffer,  which is safer
        int result = sscanf(buffer, "%d,%d,\"%29[^\"]\"", &year, &score, movie);
        // == is not safe
        if (strcmp(movie, "New_York,_New_York") == 0) {
            printf("%d", score);
            
            break;
        }
    }
    free(buffer);
    fclose(fp);
    
    return;
}

Process memory layout

OS Kernel Space
Stack
...
Heap
BSS
Data
Text
...

OS Kernel Space

Reserved for the operating system kernel and device drivers.

User processes cannot access this space directly (for protection and stability).

Contains critical code and data structures needed to manage the system.

Stack

~~~Grows downwards in memory (toward lower addresses).~~~

Stores function call information: return addresses, local variables, and function parameters.

Managed automatically by the compiler and runtime.

Each thread in a process gets its own stack.

Heap

Grows upwards in memory (toward higher addresses).

Used for dynamically allocated memory (e.g., via malloc in C, new in C++).

Managed by the programmer (or garbage collector in managed languages).

Can fragment over time due to frequent allocations and deallocations.

BSS (Block Started by Symbol)

Contains uninitialized global and static variables.

Typically zero-initialized at program startup.

For example: static int count; (without an initializer).

Data Segment

Contains initialized global and static variables.

Divided into:

Read/write section: for variables that can change during execution.

Read-only section: for constants that must not change.

Example: static int count = 5;

Text Segment (Code Segment)

Stores the compiled program instructions (machine code).

Usually marked as read-only to prevent accidental modification of code.

May be shared among processes running the same program to save memory.

How to use vim

Press Esc (goes back to Normal mode).
Then type one of these commands (they start with :):
:w → Save (write) the file.
:q → Quit.
:wq → Save and quit.
:q! → Quit without saving.

Arrow keys → work as expected.
Or use the classic Vim keys:
h → move left
l → move right
j → move down
k → move up

Moving by words
w → jump forward to the beginning of the next word
e → jump to the end of the current/next word
b → jump backward to the beginning of a word

Moving by lines
0 (zero) → move to the start of the line
^ → move to the first non-space character of the line
$ → move to the end of the line

Moving by text blocks
gg → go to the top of the file
G → go to the bottom of the file
:n → jump to line n (example: :15 goes to line 15)

Searching
/word → search forward for “word”
?word → search backward for “word”
n → repeat the last search in the same direction
N → repeat the last search in the opposite direction

Scrolling
Ctrl + d → scroll half a page down
Ctrl + u → scroll half a page up
Ctrl + f → scroll one full page down
Ctrl + b → scroll one full page up

Notes